PRML
PRML (the documents that define the markup language) is licensed under the GNU Free Documentation License, the GNU Lesser General Public License, and the BSD License.
Portions of the code are excerpted from various W3C® documents, are the property of their respective copyright holders, and are generally under either the W3C® Document License or the W3C® Software Notice And License.
Perameles
Perameles, the program, is licensed under the GNU Public License and the BSD License.
Multilicensing
This means that, except for those portions of PRML which are the property of W3C®, if you modify the code for your own purposes, you may consider it to be under any or all of those licenses. While I would prefer that any such would be released under all the same licenses which with it was obtained, this is not required. For example, you could release a modification of PRML under the LGPL if you prefer to treat your release in the manner of a software library, or under the GFDL if you consider it to be the documentation of a protocol, or under the BSD license if you want to let anyone modify what you've done without necessarily sharing their own changes.
For end users, the gist of this is that you can obtain the end program or its source code freely, and may pass it along to others as long as at least one of the licenses remains intact.
